Process for recording holographic information

ABSTRACT

A method for recording spatially modulated optical data and for the recording of holographic information in real time, comprising preparing a photosensitive recording medium comprising a polymeric sheet containing a photosensitizer adapted to bring about a change of the refractive index of the polymer upon irradiation in the visible or in the UV range and recording the information by means of a holographic set-up.

This is a division, of application Ser. No. 628,752, filed Nov. 4, 1975.

FIELD OF THE INVENTION

The present invention is in the field of coherent optics and isspecifically directed to phase storage materials for recording spatiallymodulated optical data and for recording holographic information. Unlikephotographic emulsions which record holographic data as absorptionfluctuations, phase materials record the data as index of refractionfluctuation, thereby increasing the diffraction efficiency of thehologram. In addition, the present invention deals with self-developingmaterials that need no wet chemical processing, obtaining the desiredrecording in virtually real time. Furthermore, the subject materialspossess high resolution capabilities and exceptional storage capacity.

SUMMARY OF THE INVENTION

It is the objective of the present invention to provide a novel processfor recording. It is a further object of the invention to provide aholographic system with almost instantaneous development which can beachieved without removing the recording material from the opticalsystem.

The above objects may be accomplished in accordance with the presentinvention by recording a phase hologram on a polymeric material whichwill change its index of refraction when irradiated with actinicradiation. The changes of index of refraction correspond with theoptical interference pattern to be recorded as the phase hologram.Altough the holographic technique is a preferred method for forming theinterference pattern to be recorded in the polymeric material, anysuitable optical system capable of yielding the phase hologramconstruction exposure is suitable - for example, the use of grating tomodulate an optical transparency, or the modulation of a laser beam tointroduce a carrier frequency.

BRIEF DESCRIPTION OF THE DRAWING

The drawing shows a schematical view, not according to scale, off aholographic recording arrangement of the invention.

DESCRIPTION OF THE DEVICE ACCORDING TO THE INVENTION

As shown in the drawing, light from a laser 1 (other coherent lightsources are also suitable) is divided by a beamsplitter 2; one pathconstitutes the reference beam 10 and the other constitutes the signalbeam 11. The mirrors 3 and 4 respectively, are used to reflect the saidlight beams so that they strike the recording material, 9. The pinholeassemblies 5 and 6, and the collimating lenses 7 and 8 are used toexpand and collimate the laser beams. The light after lens 7 passesthrough the transparent object 12 and interferes with the reference beam10 and the resulting interference patterns are recorded in the recordingmaterial 9. The spatial frequency of the interference fringes,determined by the angle, 0, is called the offset frequency of thehologram. Although a transparent object is shown, it is of course to beunderstood that an opaque three-dimensional object may be employed withthe light reflected from it onto the recording materials.

The set-up shown in FIG. 1 can be used in a holographic document storageand retrieval system to record documents. The recording material wouldbe partitioned for such application to many sub-areas, each comprising ahologram of a specific document. With proper apertures and indexing, theinput transparent object (which is an optical transparency of adocument) as well as the recording material, allows the recording ofanother hologram of a different document at an adjacent location on therecording material.

Alternatively, the set-up shown in FIG. 1 can be used to record complexfilters by introducing a lens into the signal path, midway between thetransparent object and the recording material -- the distance from thelens to the hologram being the focal length of the lens. The advantageof using a polymeric material over conventional photographic emulsionsin such a set-up is that the complex filters are generated in virtuallyreal-time and can be immediately employed to process incoming data. Forexample, in real-time coherent radar processing, in real-time targetrecognition and in real-time image enhancement.

In the case of three-dimensional opaque objects, a modified set-up ofFIG. 1 can be used for real-time in-situ holographic non-destructivetesting.

The reconstructed image may be viewed immediately after recording, byblocking the signal beam and viewing the image through the recordedhologram. The image will coincide in position with the original object.

A unique feature of the invention is that the reconstructed image may beviewed while the hologram is being recorded, and the recording stoppedat any desired instant. This is accomplished by recording the hologramwith a blue line (488 nm) or green line (514.5 nm) from an argon laser,and reconstructing the hologram with the red line (632.8 nm) from ahelium neon laser. With a suitable sensitizer, the polymeric mateial isnot sensitive to red illumination, so that the recorded hologram may besafely viewed to obtain a red reconstructed image while simultaneouslyrecording at a lower wave length.

the most widely described photosensitive system ispolymethylmethacrylate (PMMA) sensitized by p-Benzoquinone (PBQ). Thismaterial is prepared by introducing PBQ into PMMA through a solvent suchas chloroform, then a film is cast on a glass substrate and the solventis evaporated.

We developed a new class of photosensitive materials by introducing PBQand other sensitizers into 2-cyanoacrylate, CH₂ ═C(CN)COOR monomers andthen polymerizing the above solution as a film or sheet between twoplates. The above procedure offers great advantages both in preparationand in characteristics of the new photosensitive materials (PSM). Theuniqueness of these PSM follows from the very high reactivity of the2-cyanoacrylate monomers which therefore can polymerize very rapidlyeven after the introduction of sensitizer like PBQ. The above sensitizeris in general a very powerful stabilizer and would in most casesprohibit polymerization to occur. The solubility of the above sensitizerin 2-cyanoacrylate monomers, and the rapid polymerization that can beinduced in the above systems provide both new procedures and newmaterials in the domain of phase photosensitive materials.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention is further described by way of example only in thefollowing Examples:

EXAMPLE 1

A 10% by weight solution of PBQ in methy 2-cyanoacrylate was preparedand a few drops of the above solution were deposited on a microscopeslide. This slide was then covered by another slide and the abovesolution was pressed between these bioslides for two minutes. Thiscaused a rapid polymerization of the above monomeric solution to occur.A yellowish transparent polymer film of about 50 μ in thickness wasobtained. The above film is photosensitive in the sense that its indexof refraction is changed by impinging light on it in the spectral range380-550 nm. In particular the following experiment was performed. Asimple holographic grating was photoinduced in the above film using the488 nm line from an argon laser at an offset angle of θ = 30° . Theexperimental set-up was similar to that shown in FIG. 1.

The grating induced in this manner in the photopolymer had a diffractionefficiency of 50% for an exposure sensitivity of 3 Joules/cm². Theoffset angle θ was then increased to 120° and a new holographicrecording was made. The diffraction was comparable to that at lowerangles indicating that the resolution of the PSM exceeds 3000 lines/mm.Holographic grating was also induced in the photopolymer at 514.5 nm.

EXAMPLE 2

A 10% by weight solution of PBQ in ethyl 2-cyanoacrylate was preparedand polymerized in the manner described in Example 1. The material isphotosensitive in the range of 380-550 nm. Holographic grating wasphotoinduced in this material by the procedure described in Example 1.

EXAMPLE 3

A 0.6% by weight solution of PBQ in methyl 2-cyanoacrylate was preparedand polymerized using the procedure of Example 1. The material isphotosensitive in the sense of Example 1.

EXAMPLE 4

A 20% by weight solution of PBQ in methyl 2-cyanoacrylate was preparedand polymerized using the procedure of Example 1. The material wastested for photosensitivity and was found to be photosensitive in thesense of Example 1.

EXAMPLE 5

A 0.6% by weight solution of PBQ in ethyl 2-cyanoacrylate monomer wasprepared and polymerized using the procedure of Example 1. The materialwa tested for photosensitivity and was found to be photosensitive in thesense of Example 1.

EXAMPLE 6

A 20% by weight solution of PBQ in ethyl 2-cyanoacrylate monomer wasprepared and polymerized using the procedure of Example 1. The materialwas tested for photosensitivity and was found to be photosensitive inthe sense of Example 1.

EXAMPLE 7

Photosensitive polymers were prepared using PBQ as a sensitizer and asolvent comprising of 50% by volume solution of methyl 2-cyanoacrylatein ethyl 2-cyanoacrylate. The polymerization procedure and amount of PBQwere the same as in Example 1.

We claim:
 1. A process for recording spatially modulated data and forthe recording of holographic information in situ and in real time bymeans of a device adapted to record a hologram on a recording medium,comprising means for effecting an exposure of said recording medium withan information beam from an object illuminated with spatially coherentelectromagnetic waves and means for simultaneously exposing saidrecording medium to a reference beam of said spatially coherentelectromagnetic waves to produce on said recording medium a holograminterference pattern between said information beam and said referencebeam, said recording medium consisting of a polymeric layer of from somemicrons thickness to a few millimeters thickness of a lower-alkyl2-cyanoacrylate containing p-benzoquinone as photosensitizer, saidprocess comprising:exposing said recording medium with an informationbeam from an object illuminated with spatially coherent electromagneticwaves; and simultaneously exposing said recording medium to a referencebeam of said spatially coherent electromagnetic waves to produce on saidrecording medium a hologram interference pattern between saidinformation beam and said reference beam.
 2. A process of recordingaccording to claim 1, wherein said device includes a lens positionedbetween the object, which is a photographic transparency, and the saidrecording medium, the distance between the lens and the recording mediumbeing that of the focal length of said lens, the distance of the objectto the lens also being equal to the focal length of said lens.
 3. Aprocess according to claim 1, wherein the device includes a lens whichis in such position that the object is between the lens and therecording medium so that converging waves from the lens are modulated bythe object and converge to a focus located in the plane of the recordingmedium, said beam interfering in said plane with the waves of the saidreference beam to form a complex optical filter comprising bothamplitude and phase information of the Fourier transform of the object.4. A process according to claim 1 for the recording of an archiverecord, said record consisting of at least one holographic interferencefringe patterns forming at least one micro-hologram composed of aparticular type of interference fringe pattern resulting frominterference between the information monochromatic light flux obtainedfrom the object which is of substantially larger size than the size ofthe microhologram, and a coherent reference beam to said object beam. 5.A process according to claim 1, wherein a hologram is recorded of astatic object, said object is subjected to a predetermined stress andwaves from said object subjected to the stress are made to interferewith the reconstructed image of the said object in its static state,providing means for the real-time holographic non-destructive testing ofsaid object.
 6. A process in accordance with claim 1, wherein saidcyanocrylate is methyl- or ethyl-2-cyanoacrylate polymer or a mixture ofthese.
 7. An apparatus for recording spatially modulated data and forthe recording of holographic information in situ and in real time,comprising:a recording medium; information beam exposure means foreffecting an exposure of said recording medium with an information beamfrom an object illuminated with spatially coherent electromagneticwaves; and reference beam exposure means for simultaneously exposingsaid recording medium to a reference beam of said spatially coherentelectromagnetic waves to produce on said recording medium a holograminterference pattern between said information beam and said referenncebeam; said recording medium consisting of a polymeric layer of from somemicrons thickness to a few millimeters of a lower-alkyl 2-cyanoacrylatecontaining p-benzoquinone as photosensitizer .
 8. An apparatus inaccordance with claim 7, including a lens positioned between the object,which is a photographic transparency, and the said recording medium, thedistance between the lens and the recording medium being that of thefocal length of said lens, the distance of the object to the lens alsobeing equal to the focal length of said lens.
 9. An apparatus inaccordance with claim 7, including a lens which is in such position thatthe object is between the lens and the recording medium, so thatconverging waves from the lens are modulated by the object and convergeto a focus located in the plane of the recording medium, said beaminterfering in said plane with the waves of the said reference beam toform a complex optical filter comprising both amplitude and phaseinformation of the fourier transform of the object.
 10. An apparatus inaccordance with claim 7, wherein said cyanoacrylate is methyl- orethyl-2-cyanoacrylate polymer or a mixture of these.